1. Field of the Invention
The field of art to which this invention pertains is plastic shaping and, more specifically, it involves the mechanical shaping of polyethylene terephthalate film by stretching it, first in the machine direction, followed by heating without stretching, then cross-machine direction stretching to make a heat-sealable, biaxially oriented film suitable for use in making heat-shrinkable bands.
2. Description of the Prior Art
Numerous uses for heat-shrinkable films exist, most of which have specific property requirements for a particular use. For example, many packaging uses require balanced shrinkage, as disclosed in U.S. Pat. Nos. 2,928,132 to Richards, and 3,187,075 to Seifried et al., while other uses, such as heat-shrinkable bands to seal closures, such as on bottles, require film which shrinks predominantly in one direction, as disclosed in U.S. Pat. No. 2,784,456 to Grabenstein.
Known films of this latter type have been suitable for fulfilling requirements in a most rudimentary way, but as application technology has become more refined, and requirements in use more stringent, needs for improvements have become apparent. For instance, it has been a challenging problem to provide in a single film all of the desired properties required for this type use, such as toughness coupled with proper shrinkage. Often, in obtaining sufficient shrinkage and shrinkage force around the circumference of the band to enable it to shrink around unusual neck configurations and into the innermost depressions thereof, excessive shrinkage is placed in the band in its width direction, causing wrinkles and the like during shrinkage. If the band is oriented only in the circumferential direction to help solve this problem, adequate toughness in the film is generally impossible to obtain and, further, known methods for making this type film in which the film is uniaxially oriented, as in Grabenstein, are costly to operate due to decreased production speeds.